1. Field of the Invention
The present invention relates to a flicker-reduction method and a flicker-reduction circuit to be used for an image pickup apparatus such as a video camera or a digital still camera including an XY address scanning type imaging element (imager, image sensor), which may typically be a CMOS (complementary metal oxide semiconductor) imaging element, under the lighting of a fluorescent lamp and also to an image pickup apparatus adapted to use such a method and such a circuit.
2. Description of the Related Art
When an image of a subject is picked up by means of a video camera in direct light from a fluorescent lamp energized by a commercial AC power supply, temporal fluctuations, or so-called flicker, occur in the lightness of the video signal output as a result of the image pickup operation due to the difference between the frequency (twice as high as a commercial AC power supply) of luminance change of the fluorescent lamp (change in the quantity of light) and the vertical synchronizing frequency of the camera.
For example, when an image of a subject is picked up by a CCD camera of the NTSC system (with the vertical synchronizing frequency of 60 Hz) under the lighting of a non-inverter type fluorescent lamp in a geographical area where the frequency of the commercially supplied AC is 50 Hz, the exposure value of each pixel changes every field as shown in FIG. 1 of the accompanying drawings because of that a field period is 1/60 seconds while the period of luminance change of the fluorescent lamp is 1/100 seconds and hence the timing of exposure of each field is shifted relative to the luminance change of the fluorescent lamp.
The timing of exposure relative to the luminance change of the fluorescent lamp returns to the original one in every three fields and therefore the change of lightness is cyclic and repetitive with a period of three fields. In other words, the luminance ratio of each field (how flickering appears) changes with the exposure period but the period of flicker does not change.
However, the change of lightness in every three frames is repeated with progressive type cameras such as digital cameras when the vertical synchronizing frequency is 30 Hz.
In the case of an XY address scanning type imaging element, which may typically be a CMOS imaging element, the timing of exposure of each pixel is sequentially shifted from that of the preceding pixel in the horizontal direction by a clock (pixel clock) period and hence the timings of exposure of all the pixels differ from each other so that flicker arises in each frame and recognized as a pattern of black strips in the image. In other words, there arises a seriously degraded image.
Techniques have been proposed to reduce flicker under the lighting of a fluorescent lamp that arises in video signals coming from an XY address scanning type imaging element by extracting still parts where no moving subject exists, detecting flicker from the extracted areas and correcting the flicker (see, for example, Patent Document 1: Jpn. Pat. Appln. Laid-Open Publication No. 2001-119708).